1. Field of the Invention
The present invention relates generally to a material to be used as a barrier for containing radioactive wastes. In particular, the present invention concerns an improved backfill for an engineered barrier used in land disposal of radioactive wastes.
2. Description of Prior Art
The radioactive waste of nuclear power stations and other nuclear industry is potentially very harmful to mankind. The goal of a radioactive waste containment system is to contain and isolate the waste from the accessible environment for a long enough period of time to let the radioactive elements decay to a safe state (e.g., 1,000 years). The current trend of policies aimed at radioactive waste disposal favors geological disposal in landfills. For the purpose of absolute safety in the final disposal, the concept of a multi-barrier system for radioactive wastes has been established.
Backfill plays an important role in multi-barrier systems for the shallow land disposal of low-level radioactive wastes and for deep disposal of high-level radioactive wastes. Basically, the purposes of backfill are twofold: (1) to assist or enhance the ability of the waste containment system to isolate radioactive wastes; and (2) to function independently as a barrier to radionuclide migration should failure of the waste containment system occur.
It is envisioned that a backfill can fulfill the first purpose by controlling groundwater ingress and modifying the near-field groundwater chemistry to reduce the likelihood of waste container corrosion. The backfill can fulfill the second purpose (i.e., acting as an independent barrier to radionuclide migration) by possessing high radionuclide "sorptive" properties, by reducing near field groundwater flow rates to mitigate radionuclide transport away from the repository, and by controlling groundwater pH and Eh to keep multivalent radionuclides in their most favorable valence state for sorption.
In order to accomplish the functions outlined above, the backfill must possess appropriate properties. It is these properties that are the focus of the present invention. At the present time, there is no clearly accepted design basis for backfill. In fact, the role of the backfill component in the repository system, and moreover its effect on system performance, is not yet very well established.
It should be pointed out that the desired properties of backfill are often interdependent. For example, the reduction of groundwater flow to the waste package and the reduction of radionuclide transport are both dependent upon the permeability of the backfill.
In light of the multiple demands placed upon backfill, a blend of two or more materials are usually required to create an effective barrier, each material performing a particular function such as water flow control, near field chemistry control, mechanical strength, or radionuclide sorption control. With respect to these functions, the water flow control (water permeability) and mechanical strength are considered the most important.
In order to create a backfill barrier with low water permeability, high swelling pressure, and plasticity, researchers have looked to the expandable clays, such as the smectites. Because of their low permeability and their ability to seal fissures, smectites are perhaps the most crucial elements in current backfill designs. In addition, the use of clays in backfill provides an additional benefit because their relatively high ion-exchange capacities aid in radionuclide sorption and retardation. Clays also provide a mineral that is very important in minimizing container corrosion, and enables the multivalent actinides and technetium to be maintained in their reduced, most sorbable form.
The use of quartz sand as a major component in backfill is also known in the art (e.g., in the Swedish KBS program). While the Swedes have deemed that highly compacted bentonite clay is an adequate backfill for use in the disposal of spent reactor fuel rods, the thermal output of HLW poses a special problem. As a result, a modified backfill composed of from 80-90% quartz with the balance bentonite, has been proposed in order for the backfill to possess sufficient thermal conductivity. As a secondary benefit, the use of quartz adds to the mechanical strength of the fill. However, while the lower percentage of clay in the clay/sand mixture still lends an adequate degree of plasticity to the backfill, the overall sorptive capacity of the backfill is significantly diminished.
Bentonite/sand mixtures are recognized to be the most promising backfill material. In general, bentonite has low water permeability due to its high swelling pressure. It also has a high cation exchange capacity. On the other hand, sand has high heat conductivity, high dynamic strength, and is relatively inexpensive. A need exists for backfill materials possessing the characteristics of both bentonite and sand.
The biggest disadvantage associated with a mixture of bentonite and sand is the increased permeability in comparison with pure bentonite. For example, an experimental test shows that pure bentonite has a water permeability of 2.times.10.sup.-11 cm/sec, while a mixture of 20 wt % of bentonite and 80 wt % sand has a permeability of 9.times.10.sup.-8 cm/sec. The latter is about 5000 times greater than the former. That means the rate of the transferring of nuclides from the disposal site system through the bentonite/sand backfill would be 5000 times higher than the rate through a pure bentonite backfill. Thus, the increased effective radioactive exposure to the public in a bentonite/sand backfill limits the value of such system.
At the present time, pure bentonite clay is the most preferred backfill material and is commonly used in repository sites all over the world. Other conceptual designs include high performance concrete walls and filling materials with low permeability. While these designs may eventually evolve into effective components of advanced artificial barriers for radioactive wastes, their feasibility at the present time is limited.
Therefore, a need exists for a low-cost backfill for use in land disposal of radioactive wastes that has all of the advantages of both clay and sand, without the inherent disadvantage of increased water permeability normally associated with such mixtures.